Be it known that we, Jeffrey C. Cho, residing at 3 Caflin Farm Lane, Northborough, Mass. 01532 and being a citizen of the United States; and John F. Zettler, residing at 62 Barton Road, Stow, Mass. 01775 and being a citizen of the United States, have invented a certain new and useful BUS-POWERED DEVICE THAT INCLUDES A BUS CURRENT MODULATOR CIRCUIT INTERCONNECTED IN SERIES WITH A SIGNAL PROCESSING CONTROL CIRCUIT of which the following is a specification:
This invention relates to an improved bus-powered device and more particularly to such a device which requires less power.
In conventional bus-powered systems such as Field Bus systems a number of devices powered by the bus are serviced by a host computer. Each device may have associated with it one or more devices that transform one form of energy into another e.g. sensors for temperature, pressure, strain or acoustic transmitters or receivers or one or more actuators for valves and the like. These devices typically have two parts: the signal processing and control circuit which interfaces with the sensors or actuators and transforms their output into a digital signal for transmission to the host computer over the bus and a bus current modulator circuit which transmits those digital signals and broadcasts them on to the bus to the host computer.
In a typical case, the two circuits are connected in parallel to the bus. The signal processing control circuit requires a steady predetermined operating current while the bus current modulator circuit requires current only when communicating. But since the communication is usually a bipolar signal, such as a Manchester code, the bus current modulator circuit must be supplied at all times with at least half the current it needs only when communicating. Thus, half that current is wasted. Yet it must be available and be considered when calculating the number of devices that can be loaded on a given bus. For example, for a case in which the steady operating current for the signal processing control circuit is 16 ma and the bipolar signal used by the bus current modulator circuit swings 10 ma in each direction, the total current required is at least 10 ma plus 16 ma or a minimum of 26 ma.
It is therefore an object of this invention to provide an improved bus-powered device which uses less current.
It is a further object of this invention to provide such an improved bus-powered device which permits a greater number of devices to be served by a bus under a given power constraint.
It is a further object of this invention to provide such an improved bus-powered device in which the operating current for the signal processing control circuit is the total current of the entire device including the signal processing control circuit and the bus current modulator circuit.
It is a further object of this invention to provide such an improved bus-powered device which reduces the size of the power supply required to operate the bus.
It is a further object of this invention to provide such an improved bus-powered device which may reduce power requirements by up to 50%.
It is a further object of this invention to provide such an improved bus-powered device which even in hazardous areas with current limited by intrinsic safety barriers can increase the number of device serviceable by the bus.
The invention results from the realization that a more efficient lower power bus-powered device can be achieved by placing the signal processing control circuit and bus current modulator in series with the bus so that with the bipolar communication signal having an average current of essentially zero superimposed on the operating current of the signal processing control circuits the total current of both circuits is merely the operating current of the signal processing control circuit.
This invention features a bus-powered device including a signal processing control circuit connected to and powered by a bus at a predetermined operating current. A bus current modulator circuit is interconnected in series with the signal processing control circuit to the bus providing a bipolar communication signal. The total operating current is the greater of the current required by the signal processing control circuit or the largest absolute negative current excursion during communication.
In a preferred embodiment, the signal processing control circuit may include a power supply powered from the bus. The signal processing control circuit may include a digital signal processing circuit. The signal processing control circuit may include an analog signal conditioning and control circuit. The signal processing control circuit may include at least one sensor and/or it may include at least one actuator. The digital signal processing circuit may include a microprocessor. The bipolar communication signal may be a Manchester code. The signal processing control circuit may include a reservoir capacitor for maintaining the predetermined operating current during the low part of the bipolar communication signal. The signal processing control circuit may include a voltage regulating device controlling the voltage on the reservoir capacitor. The bipolar communication signal may have an average current of substantially zero.